Ferrofluidic rotary seals have been widely used in vacuum applications over the past 20 years. The basic structure of a conventional ferrofluid seal is illustrated in FIG. 1, which is a partial cross-sectional view of an axially symmetric seal. The seal 100 comprises a non-magnetic housing 102 through which a magnetically permeable rotary shaft passes 104. The housing 102 is closed by a non-magnetic end cap 106 that is fastened to the housing 102 by a plurality of clamping screws of which screws 103 and 110 are shown. A pair of ferrofluid seals is formed of an outer annular pole piece 112 fabricated of magnetically permeable material and an inner annular pole piece 114 also fabricated of magnetically permeable material. Pole pieces 112 and 114 are separated by an axially-polarized toroidal magnet 116. The magnet 116, the poles 112, 114 and the shaft 104 form magnetic circuits with one or more air gaps between the inner surfaces 115 and 117 of the poles 112, 114, respectively and the shaft 104. As shown in FIG. 1, the shaft may be machined to form rings or teeth near the pole pieces 122, 144. Ferrofluid 119 is attracted to each air gap and, is well known, forms dynamic O-ring seals between the pole 112 and the rotary shaft 104 and the pole 114 and the rotary shaft 104. The ferrofluid used in these seals is typically a colloidal dispersion of magnetic particles covered with a surfactant and suspended in a liquid carrier fluid such as oil. A seat is formed between the pole pieces 112, 144 and the housing 102 by rubber O-rings 118, 120, respectively at the radial interface between the parts.  
The seal embodiment shown in FIG. 1 also includes bearings 122, 124 that are also located inside the housing 102. Bearing 122 is separated from pole piece 114 by an annular spacer 126. Similarly, bearings 122 and 124 are separated from each other by annular spacer 134 and from end cap 106 by annular spacer 136. 
The seal 100 can be attached to a vacuum chamber (not shown) by bolts (not shown) that pass through holes 130 and 132 located in a flange 128 that is integral with the housing 102. 
Seals with the above structure have been effective for a variety of applications, such as semi-conductor manufacturing, optical coating, rotary gas unions, etc. However, in recent years, an increased number of applications require an ultra-high vacuum (UHV) environment. To achieve an ultra-high vacuum environment, the gas load, or the total leakage/permeation through seals plus gas evaporated inside the vacuum chamber, has to be minimized. The aforementioned rubber O-ring seals 118 and 120 typically have relatively high gas permeation rates (between 10-4 and 10-7 std cc/sec) and thus are not suitable for UHV applications. In addition, rubber O-rings, such as O-rings 118 and 120 typically have operational temperature limits less than 200° C., and, therefore, are unable to survive the high temperature baking procedure commonly used in UHV process to complete outgassing of the vacuum chamber. 
It is possible to achieve a high-quality static seal between the pole pieces 112 and 114 and the housing 102 by welding the pieces together. However, welded joints frequently cause difficulties in manufacturing, assembly, and maintenance of the seals. 